1. Field of the Invention
The present invention relates to a catadioptric reduction optical system suitably applied to a projection optical system for reduction projection in a projection exposure apparatus of a one-shot exposure method or a scanning exposure method, used to manufacture a semiconductor element or a liquid crystal display element in a photolithographic process and, more particularly, to a catadioptric reduction projection optical system having a magnification of about 1/4 to 1/5 with a resolution on the submicron order in the ultraviolet wavelength range.
2. Related Background Art
In fabricating semiconductor devices or liquid crystal display devices, etc. by photolithography process, the projection exposure apparatus is used for demagnifying through a projection optical system a pattern image on a reticle (or photomask, etc.) for example at a ratio of about 1/4 to 1/5 to effect exposure of the image on a wafer (or glass plate, etc.) coated with a photoresist and the like.
With the recent increase in the integration degree of semiconductor elements and the like, a higher resolution is required for a projection optical system used in a projection exposure apparatus. In order to meet this requirement, the wavelength of illumination light (exposure wavelength) for exposure must be shortened, or the numerical aperture (NA) of the projection optical system must be increased. If, however, the exposure wavelength is shortened, the types of optical glass which can be used in practice are limited because of the absorption of illumination light. In particular, as the exposure wavelength becomes 300 nm or less, only synthetic quartz and fluorite can be used in practice as glass materials.
The difference between the Abbe constants of the synthetic quartz and the fluorite is not large enough to correct chromatic aberration. For this reason, if the exposure wavelength becomes 300 nm or less, and a projection optical system is constituted by only a refracting optical system, chromatic aberration correction is very difficult to perform. In addition, since fluorite undergoes a considerable change in refractive index with a change in temperature, i.e., has poor temperature characteristics, and involves many problems in a lens polishing process, fluorite cannot be used for many portions. It is, therefore, very difficult to form a projection optical system having a required solution by using only a refracting system.
In contrast to this, attempts have been made to form a projection optical system by using only a reflecting system. In this case, however, the projection optical system increases in size and requires aspherical reflecting surfaces. It is very difficult to manufacture large, high-precision, aspherical surfaces
Under the circumstances, various techniques have been proposed to form a reduction projection optical system by using a so-called catadioptric optical system constituted by a combination of a reflecting system and a refracting system consisting of optical glass usable in relating to the exposure wavelength to be used. As an example, a reduction projection exposure apparatus including a catadioptric projection optical system having a beam splitter constituted by a cubic prism and serving to project a reticle image entirely by using a light beam near the optical axis is disclosed in, e.g., U.S. Pat. Nos. 4,953,960, 5,220,454, 5,089,913, or 5,402,267.